1. Technical Field of the Invention
This invention relates generally to the removal of carbon deposits in various reactors. More specifically, this invention relates to an apparatus and a method for carbon removal (or carbon ablation), which is deposited on the inner wall surfaces of reactors wherein thermal reactions take place.
2. Background of the Invention
Carbon deposits are commonly seen in reactors, especially those utilizing hydrocarbons or fuels to generate thermal energy. For example, when a stream of incompletely burned atomized fuel droplets reaches the wall of a combustor in a gas turbine engine, a localized reducing atmosphere is created. This enables carbon deposits to form on the combustor wall. Periodic breaking off of pieces of these carbon deposits can result in significant erosion damage to the combustor in the gas turbine engine.
Another case is the production of acetylene, wherein natural gas is reacted at high temperatures to produce acetylene. These high temperatures can be produced through the reaction of a fuel, such as syngas and oxidant such as oxygen. The production of acetylene, when in high enough concentration can lead to formation of carbon-containing solids in the thermal reactor in the section downstream of the hydrocarbon gas inlet. The carbon-containing solids can accumulate on the internal walls of the thermal reactor and impede flow through the reactor, resulting in increased differential pressure and/or space velocity. In some cases wherein the reacting gases pass through a restriction in the reactor, accumulation of carbon deposits in this region can impede the reaction or even stop the reaction because no flow is allowed to pass through.
Some reactors are equipped with carbon deposit removal apparatus. But when the reactor operates at temperatures above which the materials of the removal apparatus are subject to mechanical or structural degradation, the apparatus cannot remain in the reaction zone of the reactor.
U.S. Pat. No. 4,243,633 discloses a closed reactor for the thermal cracking of heavy oils, having an internally mounted, rotatable injection pipe. The injection pipe is adapted to spurt preheated raw material under pressure against the inner wall surfaces of the reactor while rotating to remove carbon which has deposited on the reactor walls during the previous cracking operation. The injection pipe is inserted into the reactor through a top opening in the reactor. The injection pipe has two axially extended portions offset from one another, one of which is located on the axis of the reactor and the other extends along an interior wall surface of the reactor at a closely spaced distance from the wall surface. The injection pipe is provided with a multitude of spouting jets longitudinally spaced along its length, each of the spouting jets is formed in the wall of said injection pipe at an angle of 25° to 90° with respect to the longitudinal axis of the injection pipe.
U.S. Pat. No. 4,224,108 discloses a carbon removal apparatus that is suitable for use on a reaction vessel for the thermal cracking of heavy petroleum oils. This apparatus essentially includes a rotatable main injection pipe to be disposed in the reaction vessel and has a multitude of jet nozzles along its length, and a second or auxiliary injection pipe positioned in the proximity of the main injection pipe to inject a scrubbing liquid over the outer peripheral walls of the main injection pipe to prevent deposition of carbon. The main and auxiliary injection pipes are both supplied with heavy petroleum oil to remove the carbon deposition from the reactor wall by the heavy petroleum oil jets from the main injection pipe while wetting the exterior of the main pipe with the heavy petroleum oil injected by the auxiliary injection pipe.
U.S. Pat. No. 4,127,473 discloses a method for the batch thermal cracking of heavy oils, such as steam blowing for production of binder pitch. The method employs a reactor having a rotary injection pipe which is rotatable within the reactor. Upon completion of the thermal cracking and withdrawal of the reaction product, the injection pipe ejects preheated raw material under pressure against the interior wall surfaces of the reactor while in rotation to remove carbon which has deposited on the reactor walls during the previous cracking operation.
U.S. Pat. No. 4,917,787 utilizes a reactor with an insulator. Insulators are physically weak materials compared to metals and are subject to ablation, erosion, cracking and other physically degenerative mechanisms when exposed to flowing gases, flowing gases that contain solids, liquid sprays, liquid sprays that contain solids, and other flowing media. In addition, insulators generally have porous surfaces compared to metals onto which carbon deposits can attach and intrude into, making removal of said carbon deposits difficult or impossible without abrading or otherwise damaging the insulating surface and insulator integrity.
U.S. Pat. Nos. 3,557,241 and 3,365,387 disclose the introduction of sufficient steam and/or water to at least one tube of the cracking furnace while simultaneously reducing the hydrocarbon feed to that tube. The tube is then put back into service. The treatment of the tube is effected at temperatures ranging from as low as 370° C. (700° F.) to about 1100° C. (2000° F.). Such heat is supplied by external firing of the reactor tubes. Both Patents utilize a separate and distinct feed line for introducing steam and/or water for the on-stream carbon removal procedure. These lines are controlled by a valve which is put into service on only those occasions when the individual tube in question being subjected to carbon removal is undergoing such a cleaning operation.
U.S. Pat. No. 3,920,537 deals with the carbon deposition evolving from hydrocarbon cracking operations by “periodically contacting the carbon deposit with a jet of relatively cold, high-pressure water.” The Patent describes jetting the high-pressure cold water against the carbon deposit in an amount sufficient to thermally shock and break up the carbon deposit, typically at a pressure in excess of about 5000 pounds per square inch. This type of carbon removal technique, however, is only particularly useful where the carbon deposition occurs on surfaces having temperatures of approximately 370° C. (700° F.) to 538° C. (1,000° F.).
U.S. Pat. No. 4,203,778 effects carbon removal of furnace tubes by the use of a turbulent stream of impact resistant, non-angular, non-abrasive particles entrained in a gas stream. The particles are entrained at a concentration of 0.1 to 1.0 pound per pound of gas and the gas is introduced into the inlet end of the furnace tubes at a gas flow rate corresponding to an inlet velocity of 14,000 to 20,000 feet per minute.
Generally speaking, the prior art carbon removal procedures in the hydrocarbon cracking field, operate under certain process constraints. The prior art utilizes carbon removal procedures wherein the reactors are made of metal. These processes are operated at reaction temperatures not exceeding about 1100° C. Because the reactors are made of metal, the heat for the carbon removal reactors is transferred through the walls. They usually require taking the reaction train equipment out of service and specially treating that equipment so as to reduce or eliminate the coking problem. Furthermore, in most cases, these processes require the dismantling of equipment or the addition of equipment in order to effect carbon removal. Such procedures are exceedingly time consuming, and add materially to the cost of the operation of the hydrocarbon cracking apparatus.
U.S. Pat. No. 4,849,025 uses an oxidant to oxidize the carbon and uses a liquid to take carbon away. Using an oxidant in this manner is a departure from normal operating conditions under which the reaction is meant to proceed. Changing the contents or stoichiometry in the reactor reduces conversion to the desired product and often the product is not collected during this non-production stage. Also, changing operating conditions of the reaction results in non-normal control states; thus non-normal function required for process control devices such as control valves, flow meters, temperature indicators, pressure indicators and similar devices, complicates process control and the return to normal operating conditions.
A mechanical apparatus to physically dislodge or scrub carbon particles has been employed in some reactors. U.S. Pat. No. 4,196,050 (1980) of Takahashi et al. describes a rotatable injection pipe for introduction of a scrubbing liquid with means for reciprocating motion.
U.S. Pat. No. 4,673,442 uses a physically reaming device to remove a bed of carbon from vessels used to produce carbon. Use of physical devices to remove carbon deposits imparts shear and normal stress to the carbon that are imparted to the reaction vessel which can damage or weaken the vessel, reducing its useful life. In addition, such devices can directly deform, crack, or otherwise damage the surface when the device directly impacts the reactor vessel surface.
U.S. Pat. No. 4,626,320 utilizes hydro-blast drilling to remove petroleum carbon from delayed coking drums. Use of hydroblast drilling is disadvantageous because of the damage that very-high-velocity spray can have on reactor internals and components.
U.S. Pat. No. 4,297,147 utilizes abrasive particles added to gas at high gas velocity for carbon removal. Use of abrasive particles leads to erosion of metal surfaces and reduces the useful lifetime of reaction vessels due to the scoring action that abrasive particles have at high velocities. Also, high velocity particles tend to cause greater damage at bends, angles, corners and other direct or glancing impact locations.
Several methods for internal cleaning or carbon removal of hydrocarbon furnace tubes are currently employed, the most common of which are mechanical cleaning (commonly known as turbining), hydroblasting, and stream-air carbon removal.
Turbining essentially consists of cutting or reaming the carbon deposits from the tube wall by passing a cutting head through each straight section. This method requires that the furnace be disassembled to the extent that the inlet and outlet of each individual straight section of tube is exposed to allow entry of the cutting head. For those furnaces of welded return bend design this means that return bends must be initially cut off and welded back in place after cleaning. Commercial sandblasting is usually employed to clean the return bends. This method has several major drawbacks, including: (1) that it results in substantial downtime; (2) it is labor intensive; (3) it results in substantial tube wall wear and subsequent premature tube failure as a result of improper alignment of cutting head and furnace tube; and (4) it causes severe erosion of return bends.
The second technique, known as hydroblasting, is similar to turbining except that, instead of the cutting tool, a hydraulic device is inserted into each tube. The device produces high pressure water jets directed normal to the tube wall which dislodge the deposit by impact. Again, this method results in substantial downtime and is labor intensive for the same reasons mentioned above. Furthermore, the high pressure water tends to dissolve sulfur initially deposited on the tube wall and results in possible sulfuric acid corrosion of the tubes in addition to creating a significant waste disposal problem.
Both of the above processes require that the furnace be cooled to near atmospheric temperature. Not only does this result in significant additional downtime, but in certain furnaces the cool down process itself can result in destruction of the furnace tubes. It is not uncommon during cool down for a furnace tube to fracture longitudinally as a result of differential thermal contraction. The heavy inner layer of carbon has a significantly lower thermal expansion coefficient compared to typical tubing material and can result in circumferential thermal stresses in the tube wall in excess of its ultimate tensile strength.
Probably the most common method of carbon removal furnace tubes is by injecting metered amounts of steam and air into the tubes with the furnace fired. The solid carbon is thus removed by a highly exothermic reaction between the solid carbon and air which generates a gas-solid stream of carbon particulate, CO, CO2, SO2 and NOx. The stream is used to cool the products of reaction. Process steps include: (1) removing the furnace from hydrocarbon service; (2) connecting carbon removal lines to the furnace; and (3) introducing steam and air to induce controlled burn out. Though furnace downtime is considerably less than for the above two processes, this steam-air process can result in serious and costly furnace damage. During the process, the tube skin temperature must be maintained within very narrow limits so as to both sustain the temperature required to support the reaction and yet limit the reaction temperature below the tube melting point. This highly exothermic reaction frequently results in ruptured tubes and fittings and hence costly downtime. In addition, the high temperature reaction of oxygen can leave an oxide layer on the inner tube wall which will inhibit heat transfer. Mechanical cleaning or polishing must be used to remove the deposits subsequent to steam-air carbon removal operations. Finally, a further disadvantage of this process is that the effluent gases are highly toxic and thus create serious environmental problems, if not properly handled.
Therefore, there is continuing need and interest to develop apparatus and methods for removal of carbon deposits in various reactors.